Field of the Invention
This invention relates to organosiloxane compositions that cure by a platinum-catalyzed hydrosilation reaction. More particularly this invention relates to combinations of additives for one-part organosiloxane compositions that impart both long term storage stability and adhesion to a variety of substrates, including metals and printed circuit boards, that the composition is in contact with during curing without interfering with curing of the composition at temperatures below about 120.degree. C.
The curing characteristics of organosiloxane compositions that cure by a platinum-catalyzed hydrosilation reaction and the properties of the cured materials obtained from these compositions are desirable for a variety of end use applications in the electrical and electronics areas. The cured materials are particularly useful as protective coatings for integrated circuits and other electronic devices and as coatings for printed circuit boards.
A shortcoming of organosiloxane compositions that cure by a hydrosilation reaction is their inability to adhere strongly to many substrates, particularly plastics. One method for overcoming this deficiency is by the use of primer compositions or adhesion promoting additives containing silanes and organosiloxanes with a plurality of silicon-bonded hydrolyzable groups and at least one organofunctional substituent bonded to silicon through at least one carbon atom. Primer compositions are applied to substrates to which adhesion is desired prior to application of the curable organosiloxane composition. Adhesion promoters are an ingredient of the curable organosiloxane composition.
Examples of hydrolyzable groups are alkoxy, carboxy such as acetoxy, ketoximo such as methylethyl ketoximo, and amido. The organofunctional group is typically one that will react with the substrate to which bonding is desired. Examples of suitable organofunctional groups include but are not limited to alkenyl such as vinyl and allyl, aminoalkyl, epoxyalkyl, mercaptoalkyl and (meth)acryloxyalkyl.
Two classes of silanes frequently used as adhesion promoting additives are vinyltrialkoxysilanes and epoxy-functional silanes such as 3-glycidoxypropyltrialkoxysilane. The use of these silanes individually or in combination as ingredients of curable organosiloxane compositions is described in Japanese Laid-Open Applications 57/87402, 54/1327, 52/32030, 2/18452 and West German published application 3,828,098. U.S. Pat. No. 4,703,074 describes polyphenylene sulfide resin compositions containing the combination of a vinyltrialkoxysilane with either an aminosilane or an epoxy-functional silane to improve the moisture resistance and mechanical strength of the resin.
European patent application no. 240,333 describes organic or silicone resin compositions containing up to 100 weight percent, based on the weight of the resin, of particles obtained by curing an organosiloxane composition containing an alkoxy substituted silane such as 3-glycidoxypropyltrimethoxysilane and an aluminum or zirconium compound.
The prior art also teaches using mixtures of two or more organosilicon compounds to achieve cohesive bonding between silicone elastomers cured by a platinum-catalyzed hydrosilation reaction and various substrates while extending the working time of the corresponding curable compositions. The combination of (1) a polyorganosiloxane containing a hydroxy group and a vinyl radical and (2) an epoxy-functional silane is taught in U.S. Pat. No. 4,087,585 that issued to Schulz on May 2, 1978. This patent also discloses the necessity of heating curable compositions containing this adhesion promoter to temperatures of at least 100 degrees C. to overcome the inhibiting effect of the adhesion promoter on the curing reaction.
A disadvantage of many organofunctional organosilicon compounds used as adhesion promoters in prior art compositions, particularly those described in the aforementioned U.S. Pat. No. 4,087,585, is their adverse effect on the cure rate and completeness of cure, particularly at temperatures below about 120.degree. C. Some integrated circuits and other heat-sensitive electronic devices cannot be exposed to temperatures above this limit even for relatively short periods of time without the risk of damage to the device.
U S. Pat. No. 5,082,894, which issued to Vanwert and Houghtaling on Jan. 21, 1992, teaches that the shortcomings of poor storage stability and relatively high curing temperatures associated with the compositions described in the aforementioned U.S. Pat. No. 4,087,585 can be overcome by including in the curable composition as the catalyst inhibitor an acetylenic alcohol containing at least 8 carbon atoms. The compositions described in this patent do not adhere well to metals such as aluminum and stainless steel, and they cannot be cured at temperatures below 100.degree. C.
One objective of this invention is to provide a group of additives that can be incorporated into one-part organosiloxane compositions to achieve long term storage without sacrificing the ability of the composition to cure completely while developing strong adhesion to plastics, glass reinforced plastics and metals during relatively short exposures to temperatures below about 110.degree. C.
A second objective of this invention is to provide a class of curable organosiloxane compositions that can be used as coating materials for heat-sensitive electronic devices and circuit boards containing these devices.